Ion implantation is used in integrated circuit fabrication to accurately introduce controlled amounts of dopant impurities into semiconductor wafers and is one of the processes of microelectronic/semiconductor manufacturing. In such implantation systems, an ion source ionizes a desired dopant element gas, and the ions are extracted from the source in the form of an ion beam of desired energy. Extraction is achieved by applying a high voltage across suitably-shaped extraction electrodes, which incorporate apertures for passage of the extracted beam. The ion beam is then directed at the surface of a workpiece, such as a semiconductor wafer, in order to implant the workpiece with the dopant element. The ions of the beam penetrate the surface of the workpiece to form a region of desired conductivity.
Several types of ion sources are used in ion implantation systems, including the Freeman and Bernas types that employ thermoelectrodes and are powered by an electric arc, microwave types using a magnetron, indirectly heated cathode (IHC) sources, and RF plasma sources, all of which typically operate in a vacuum. In any system, the ion source generates ions by introducing electrons into a vacuum arc chamber (hereinafter “chamber”) filled with the dopant gas (commonly referred to as the “feedstock gas”). Collisions of the electrons with atoms and molecules in the dopant gas result in the creation of ionized plasma consisting of positive and negative dopant ions. An extraction electrode with a negative or positive bias will respectively allow the positive or negative ions to pass through an aperture as a collimated ion beam, which is accelerated towards the target material.
In many ion implantation systems, carbon, which is known to inhibit diffusion, is implanted into the target material to produce a desired effect in the integrated circuit device. The carbon is generally implanted from a feedstock gas such as carbon monoxide or carbon dioxide. The use of carbon monoxide or carbon dioxide gases can result in oxidation of the metal surfaces within the plasma source (arc chamber) of the ion implanter tool, and can also result in carbon residues depositing on electrical insulators. These phenomena reduce the performance of the implanter tool, thereby resulting in the need to perform frequent maintenance. Oxidation can result in inefficiencies in the implantation process.
Frequency and duration of preventive maintenance (PM) is one performance factor of an ion implantation tool. As a general tendency the tool PM frequency and duration should be decreased. The parts of the ion implanter tool that require the most maintenance include the ion source, which is generally serviced after approximately 50 to 300 hours of operation, depending on operating conditions; the extraction electrodes and high voltage insulators, which are usually cleaned after a few hundred hours of operation; and the pumps and vacuum lines of vacuum systems associated with the tool. Additionally, the filament of the ion source is often replaced on a regular basis.
Ideally, feedstock molecules dosed into an arc chamber would be ionized and fragmented without substantial interaction with the arc chamber itself or any other components of the ion implanter. In reality, feedstock gas ionization and fragmentation can results in such undesirable effects as arc chamber components etching or sputtering, deposition on arc chamber surfaces, redistribution of arc chamber wall material, etc. In particular, the use of carbon monoxide or carbon dioxide gases can result in carbon deposition within the chamber. This can be a contributor to ion beam instability, and may eventually cause premature failure of the ion source. The residue also forms on the high voltage components of the ion implanter tool, such as the source insulator or the surfaces of the extraction electrodes, causing energetic high voltage sparking. Such sparks are another contributor to beam instability, and the energy released by these sparks can damage sensitive electronic components, leading to increased equipment failures and poor mean time between failures (MTBF).
In another instance of undesirable deposition, various materials (such as tungsten) can accumulate on components during extended ion implantation processes. Once enough tungsten is accumulated, the power used to maintain temperature sufficient to meet the beam current setpoint may not be sustainable. This causes loss of ion beam current, which leads to conditions that warrant the replacement of the ion source. The resultant performance degradation and short lifespan of the ion source reduces productivity of the ion implanter tool.
Yet another cause of ion source failure is the erosion (or sputtering) of material. For example, metallic materials such as tungsten (e.g., the cathode of an IHC source or the filament of a Bernas source) are sputtered by ions in the plasma of the arc chamber. Because sputtering is dominated by the heaviest ions in the plasma, the sputtering effect may worsen as ion mass increases. In fact, continued sputtering of material “thins” the cathode eventually leading to formation of a hole in the cathode (“cathode punch-through” in the case of IHC), or for the case of the Bernas source, creates an opening in the filament. Performance and lifetime of the ion source are greatly reduced as a result. The art thus continues to seek methods that can maintain a balance between the accumulation and erosion of material on the cathode to prolong the ion source life.